1. Field of the Invention
The present invention relates to magnetic resonance isolators, and specifically to magnetic resonance isolators used in a microwave frequency band, for example.
2. Description of the Related Art
In general, an isolator has a characteristic of transmitting signals in a predetermined direction and not transmitting signals in the opposite direction, and is mounted in a transmitter circuit of a mobile communication apparatus, such as a cellular phone. Known examples of magnetic resonance isolators include isolators described in Japanese Unexamined Patent Application Publication No. 63-260201 and Japanese Unexamined Patent Application Publication No. 2001-326504. A magnetic resonance isolator utilizes a phenomenon in which, when high-frequency currents of equal amplitude whose phases differ by a quarter wavelength flow through two lines (with four ports) perpendicular to each other, a rotating magnetic field (circularly polarized wave) is generated at the intersection and the circulation direction of the circularly polarized wave is reversed in accordance with the propagation directions of the electromagnetic waves along the two lines. In other words, by arranging a ferrite member at the intersection and applying a static magnetic field necessary for magnetic resonance using a permanent magnet, a positive or negative circularly polarized wave is generated by a wave reflected from a sub-line in accordance with the propagation direction of an electromagnetic wave along a main line. When a positive circularly polarized wave is generated, a signal is absorbed due to the magnetic resonance of the ferrite member, and when a negative circularly polarized wave is generated, no magnetic resonance is generated, whereby the signal can pass through the intersection without attenuation. Reactance elements for reflecting the signals are connected to the ends of the sub-line.
However, such a known magnetic resonance isolator has a large size, for example, about 20 mm×about 20 mm for a frequency of about 2 GHz, since the main line is a quarter wavelength long so as to resonate and two reactance elements are mounted thereon. This is problematic in view of recent trends in mobile communication apparatuses, i.e., reduction in size and increasing component mounting density.